Conventional acoustic ceiling tile is a non-woven structure including a core composed of base fibers, fillers, and binders combined to form the ceiling tile structure. The base fibers are usually mineral wool or glass fibers. The fillers can be perlite, clay, calcium carbonate, cellulose fibers, and the like. The binders are typically cellulose fibers, starch, latex, and the like. Upon drying, the binder forms bonds with the base fibers and fillers to form a fibrous network that provides structural rigidity to the tile and forms a porous structure to absorb sound. To be used as a typical ceiling tile, the non-woven structure or base mat should be substantially flat and self-supporting in order to be suspended in a typical ceiling tile grid or similar structure.
For non-woven structures to be suitable in acoustical ceiling tile applications, the non-woven structure also needs to comply with various industry standards and building codes relating to noise reduction and fire rating. For example, industry standards require ceiling tiles to have a Class A fire rating according to ASTM E84, which generally requires a flame spread index less than 25 and a smoke development index less than 50. Regarding noise reduction, industry standards typically require the acoustical ceiling tile to have a noise reduction coefficient (NRC) according to ASTM C423 of at least about 0.55.
Acoustic ceiling tiles are commonly formed via a wet-laid process that uses an aqueous medium to transport and form the tile components into the desired structure. The basic process involves first blending the various tile ingredients into an aqueous slurry, transporting the slurry to a head box forming station, and distributing the slurry over a moving, porous wire web into a uniform mat having the desired size and thickness. Water is removed, and the mat is then dried. The dried mat may be finished into the ceiling tile structure by slitting, punching, coating and/or laminating a surface finish to the tile. In the wet-laid process, water serves as the transport media for the various tile ingredients. However, while convenient for high production speeds and the ability to use low cost raw materials (for example, recycled newsprint fibers, recycled corrugated paper, scrap polyester fibers, cotton linters, waste fabrics, and the like), using water to manufacture acoustical ceiling tile presents a number of shortcomings that render the process and formed product less than desirable.
The wet-laid process uses a great deal of water to transport and form the components into the ceiling tile structure. The large amount of water must eventually be removed from the product. Most wet processes, therefore, accommodate water removal by one or more steps of free or gravity draining, high and low vacuum, compression, and evaporation. Unfortunately, these process steps entail large energy demands to transport and to remove the water. As such, the handling of large volumes of water to form the tile along with the subsequent removal and evaporation of the water renders the typical wet-laid process relatively expensive due to high equipment and operating costs.
It also is difficult using a wet-laid process to form an acoustical ceiling tile having high sound absorption properties. In a wet-laid process, the formed ceiling tiles tend to have a sealed surface due to the nature of the ingredients in the wet-laid formulation. A ceiling tile with a sealed surface generally has a less efficient acoustical barrier because the tile is less porous, which renders the tile less capable of absorbing sound. The sealed tile surface may actually reflect sounds, which is an undesired characteristic in an acoustical ceiling tile.
These undesired acoustical characteristics are believed to occur from the hydrophilic nature of the tile ingredients typically used in the wet-laid process. Cellulose fibers (for example, recycled newsprint), which are commonly used as low cost binder and filler in a ceiling tile, are highly hydrophilic and attract an extensive amount of water. Due in part to such hydrophilic components, wet-laid tiles typically have a high tipple moisture content (i.e., the moisture level of the board immediately prior to entering the drying oven or kiln) of about 65 to about 75 percent, which increases the demands of evaporation during drying. As a result, a high surface tension is generated on the tile ingredients during drying as water is removed from these hydrophilic components. Water, a polar molecule, imparts surface tension to the other components. This surface tension generally causes the tile surface to be sealed with a less porous structure. It is believed that the surface tension draws elements in the tile closer together densifying the structure and closing the tile pores in the process. Consequently, wet-laid produced ceiling tiles require further processing to perforate the tile in order to achieve acceptable noise reduction. Therefore, while a wet-laid process may be acceptable due to increased production speeds and the ability to use low cost materials, the use of water as a transport media renders the process and resulting product less cost effective when acoustic characteristics are required for the product.
In some cases, a latex binder also may be used in acoustical ceiling tiles and is often preferred in a wet-laid process using mineral wool as the base fiber. Latex, however, is generally the most expensive ingredient employed in a ceiling tile formulation; therefore, it is desired to limit the use of this relatively high cost ingredient. Other binders commonly employed in ceiling tiles are starch and, as described above, cellulose fibers. Starch and cellulose, however, are hydrophilic and tend to attract water during processing and generate the high surface tension problems described above.
A common shortcoming of acoustic ceiling tiles fabricated using a wet-laid process is that such formed tiles generally lead to a higher density through the above described mechanism. The high density is often associated with high air flow resistivity, which compromises acoustical absorption. Typically, tiles made with a conventional formula have a density of about 12 lbs/ft3 to about 20 lbs/ft3 depending upon its composition. They also have a noise reduction coefficient (NRC) of about 0.55 to about 0.80, depending upon specific composition. For basemats with similar compositions, a lower density normally results in lower air-flow resistivity or higher porosity, thus improving acoustical absorption. However, if the composition is different the association of density with porosity is not necessarily as stated above.
Alternative bonding fibers have been developed, but such alternative fibers are still fabricated using hydrophilic components and would, therefore, exhibit the same shortcomings as found in existing ceiling tile ingredients. For example, U.S. Pat. Nos. 6,818,295 and 6,946,506 and US Publication No. 2005/0026529 describe a finely attenuated fiber having a plurality of microfibrils thereon. The inventors of these references suggest that the microfibrils mechanically reinforce a non-woven material to provide improved tensile strength. The fibers in these references, however, are still constructed using a starch matrix, which provides a natural polymer to bind the components together. The starch is important in these cases because it allows any formed material to be biodegradable. However, if the starch described in these references were used to form a ceiling tile, the formed tile would exhibit the same shortcomings as found in the wet-laid tiles because of the hydrophilic nature of the starch. That is, as described above, the starch matrix would be expected to create a high surface tension during water removal and tend to form a sealed surface lowering the ability of the tile to absorb sounds. These references further suggest that the starch matrix can be removed from the fiber structure and only the microfibrils used. In such case, however, if only the individual microfibrils without the benefit of the base fiber structure were used in a ceiling tile, they would not provide a sufficient bonding matrix and strength to function as an effective binder in a ceiling tile structure.
Accordingly, there is a desire for a low density, non-woven structure with a minimum of hydrophilic components that is flat, self-supporting, and suitable under industry standards for an acoustic ceiling tile (i.e., both thermal and acoustic properties) that meets user expectations for manual cutability.